1. Field of the Invention
The present invention relates generally to tough, high performance, thermoplastic polymers. It relates particularly to addition-type thermoplastic polymers, which are polymers having an addition curing which leads to a linear structure.
2. Description of Related Art
High performance thermosetting and thermoplastic polyimides are under extensive experimental and developmental evaluation for a large number of aerospace structural applications because of their high glass transition temperatures (T.sub.g s), excellent mechanical performance, and ability to withstand harsh environments, such as heat, moisture and chemicals. However, the poor processability and lack of damage tolerance remain the fundamental problems of these materials which seriously inhibit their widespread use in fabricating large aerospace structures. Thermosetting polyimides are, in general, easy-to-process but inherently brittle, whereas thermoplastic polyimides are tough but difficult-to-process. What is needed is a polymer that can be processed like a thermoset while possessing the toughness of a thermoplastic.
An addition-type thermoplastic (hereafter referred to as "ATT") polymer has characteristics of such an ideal polymer. An ATT is defined as a polymer that has an addition curing which leads to a linear structure. Such a polymer is non-classical in that it has similarities to two major classical categories: thermosets (addition curing with a crosslinked structure) and thermoplastics (condensation reaction cured having a linear structure). Because of their addition curing and linear structure, ATT polymers can have toughness (like thermoplastics) and can be easily processed (like thermosets).
Arnold et al (U.S. Pat. No. 4,675,370) and Kirchhoff (U.S. Pat. No. 4,540,763) and others have developed a synthetic route for making ATT polymers. Their synthesis involves the Diels-Alder reaction of a benzocyclobutene and a bismaleimide, as illustrated by the following reaction. ##STR1## The scope of this synthetic reaction is limited by the availability of benzocyclobutene compounds and other factors, such as processing, properties and cost effectiveness of the end use products.
Similarly, D'Alelio (U.S. Pat. No. 4,451,402) has also used the Diels-Alder reaction between a conjugated diyne and a bismaleimide to obtain ATT polymers. This reaction is shown below: ##STR2## Further, Grimes et al (U.S. Pat. No. 4,365,034) have developed a methodology to improve the processability of the acetylene-terminated Thermid polyimide system.
However, none of these prior art products exhibit the desired combination of properties provided by the present invention.
Accordingly, the primary object of this invention is to provide a synthetic reaction which forms stable aromatic rings in the backbone of an ATT polymer, thereby combining high temperature performance and thermo-oxidative stability with toughness and easy processibility, and minimizing or eliminating the necessity for property tradeoffs often observed in conventional polymer synthesis.
Another object of this invention is to obtain polymers having the combination of easy processability, toughness, high temperature performance and good thermo-oxidative stability in one material.
Yet another object is to develop methods for making and using these polymers.
It is another object of the present invention to provide novel monomeric materials for use in attaining the primary object above.